A prior art fuel cell is an electrochemical energy conversion device. Chemical energy stored in a fuel such as hydrogen is converted via an electrochemical reaction, with the participation of an oxidizer such as oxygen, into electrical energy of electrons flowing through an external electrical load. Operation proceeds as long as the flows of hydrogen and oxygen, and the electrolyte concentration, are maintained.
The basic concept of the prior art fuel cell is an electrochemical cycle based on a closed electrical circuit, consisting of ion transport from one electrode to the other inside the cell, together with electron flow from one electrode to the other outside the cell.
A typical reaction on the anode side is:H2→2H++2e−
The H+ ions travel through electrolyte towards the cathode, whilst the electrons, the desired product, travel in an external electrical circuit from the anode to the cathode.
On the cathode side oxygen reacts with the produced 2H++2e−, producing water:½O2+2H++2e−→H2O
The electric current flow with the complete electrochemical cycle is shown in these reactions by the anode producing two positive ions and two electrons, and the cathode receiving them.
The expressions show that the electrodes of the prior art fuel cell react mutually by transferring ions from one electrode to the other, while the electrodes remain unchanged in the cell.
In other fuel cell types the oxygen picks up electrons at the cathode and travels through the electrolyte to the anode, where it combines with hydrogen ions.
The ion transport requirement, together with electron flow blocking between the electrodes, is a challenge of prior art fuel cells. Thus fuel cells' most common classification is by the type of the electrolyte, such as Alkaline (AFC), Molten Carbonate (MCFC), Phosphoric Acid (PAFC), Proton Exchange Membrane (PEMFC), Solid Oxide (SOFC) and Direct-Methanol (DMFC).
Unlike prior art fuel cells, ion transport is neither a basic requirement nor a challenge for storage cells. The difference between traditional storage cells and prior art fuel cells is that each electrode of the storage cell independently reacts chemically with the adjoining electrolyte, rather than exchanging ions between the electrodes as in prior art fuel cells. While the electrodes of a storage cell react and chemically change as the storage cell is charged or discharged, the fuel cell's electrodes are chemically inert, apart from catalytic action.
A general fashion lead-acid storage battery is made of assembled plate groups, immersed in electrolyte in a sealed container, with + and − terminals. A plate consists of a rectangular structure of lead, alloyed with a little antimony to improve the mechanical characteristics. The plate is in fact a grid with rectangular holes in it, the lead forming thin walls to the holes. The holes are filled with a mixture paste of red lead (Pb3O4) and dilute sulfuric acid or other. The paste is pressed into the holes which are slightly tapered on two sides to assist in the retention of the paste. This paste remains porous and allows the acid to react with the lead inside the plate increasing the surface area many fold. The plates are then stacked together with suitable separators, the choice shifted from wood to rubber to glass mat to cellulose based separators to sintered PVC to microporous PVC/polyethylene separators, thereby fabricating a group of electrodes. Each set of alternate plates in a stack are connected together by a connecting conductor. At this stage the positive and negative plates are identical.
A cell refers to each stack of positive and negative plate pairs. The assembly of several cells in series to produce a higher voltage is called a battery. Lead-acid car batteries for a typical 12 volt system consist of six storage cells of 2.1 volt nominal voltage. Electrolyte is added to the battery, and the cell is then given its first forming electrical charge. The positive plates gradually turn to lead dioxide, and the negative turn to lead.
Storage batteries, such as of lead-acid type, are advantaged over prior art fuel cells for supplying high surge currents, despite low energy-to-weight ratio and a correspondingly low energy-to-volume ratio. This, along with their low cost, makes them indispensable for use in cars, as they aptly provide the high electric power required by automobile starter motors.
The prior art fuel cell types detailed above do not store electrical energy. To overcome their insufficient power output capability in some applications such as small stand-alone power plants or the so called “fuel cell battery hybrid” such as in vehicles, fuel cells are combined with electrical storage systems to form an electrical power source of sufficiently high power output capability.
U.S. Pat. No. 7,033,699 to OVSHINSKY discloses an approach to fuel cells comprising storage of energy. OVSHINSKY discloses fuel cell cathodes and instant startup fuel cells employing these cathodes. The cathodes operate through the valence change mechanism of redox couples which uniquely provide multiple degrees of freedom in selecting the operating voltages available for such fuel cells.
Such cathodes provide the fuel cells in which they are used, particularly alkaline fuel cells, with a level of chemical energy storage within the cathode itself. This means that such fuel cells will have a “buffer” or “reactant charge” available within the cathode at all times.
The cathode in accordance with the OVSHINSKY invention comprises a cathode active material including a valence change material. The valence change material provides the cathode with an oxygen storage capacity.
OVSHINSKY argues that in cathodes prior to his invention, no storage of reactant occurs. That is, oxygen travels directly through the active materials and reacts at the electrolyte interface. In the cathodes of the OVSHINSKY invention, oxygen is stored in the cathode via a change in valence state within the reversible redox couples, and is then available, at the electrolyte interface of the cathode.
A fuel cell that has the built in hydrogen and oxygen storage of the OVSHINSKY invention, has the advantage of being able to start producing energy instantly from the reactants stored in its electrodes. Thus, there is no lag time while waiting for hydrogen to be supplied from external sources. Additionally, because hydrogen and oxygen can be adsorbed and stored in the respective electrode materials of the fuel cell, energy recapture can be achieved as well. Therefore, according to OVSHINSKY, activities such as regenerative braking, e.g braking energy in electric cars, etc., can be performed without the need for a storage cell that is external to the fuel cell because any reactants produced by running the fuel cell in reverse will be stored in the electrodes of the fuel cell.
Therefore, in essence, fuel cells employing the OVSHINSKY active electrode materials would be the equivalent of a fuel cell combined with a storage cell.
However, OVSHINSKY teaches storage of hydrogen and oxygen, not of electricity as in a storage cell.
It would be desirable to provide a fuel cell in which the electrical energy is stored, similarly as it is in a storage cell; and it would be desirable to provide a storage cell, such as a lead-acid type, which is capable of being charged with electricity from an external supply of fuel and oxidizer.